This invention relates to switch mats of a type having several key switches, each of which has a dome-shaped key cap for being pressed toward a contact sheet by a key to complete a contact with a contact lug, with switch chambers under the respective key caps being connected to atmosphere by a ventilating channel. Switch mats of this type comprise individual or multiple adjacent key switches arranged as a keyboard, and are used in electrical control panels or in data entry keyboards.
A dome-shaped key cap is placed over an actuating element of each key switch; for keyboards, the key cap may be designed as a mat. Venting air out of and into a switch chamber inside the key cap is absolutely essential when the key cap is pressed. This can be accomplished by releasing the air directly to atmosphere, or by venting the air into and out of other switch chambers, with an equalizing of pressure.
German patent document (DE-38 09 770 C2) discloses a key switch of this type. A disadvantage of this system is that when air is vented directly outside, dust and moisture are drawn back into a switch chamber with an ensuing air intake.
By contrast, if the air is vented into and out of other switch chambers, as disclosed in German patent document (DE 29 45 707 C2), no dust or moisture can come in. However, because of an air cushion and a resulting change in pressure, an operating force of a key switch is modified for a switch chamber of this type, which may result in incorrect key operation.
German patent document (DE 42 12 562 A1) discloses a contact-sheet keypad with snap keys. Ventilation holes are located next to the keys to allow air to be vented out and in during keying. Air under the snap keys is not removed; rather, it remains there to act as a pneumatic spring, generating return force.
In one embodiment of German patent document (DE 90 12 407.3 U1), a minimum of two outlets must be provided in a support layer to permit pressure equalization for a press switch element. In this design, air ventilation for a gap in a contact area takes place through a filter material and the outlets. To prevent destruction by corrosion of a switching matrix, it is recommended either that an insulating break in an electrically conductive layer be provided at a periphery of a contact area, with a center contact, or that an additional desiccating agent, e.g. silica gel, be used. This is very expensive, and cannot be used for key switches of a switch mat.
It is an object of this invention to provide such a switch mat that does not allow dust and moisture to penetrate in during ventilation, while also not influencing a switching procedure.